Space is already radioactive. The vehicles using nuclear propulsion will be outside of the atmosphere and shouldn't make an impact on radiation levels on earth. It was frowned upon because it was seen as putting nuclear weapons in Space, but that's different now. We could all easily kill each other with ICBM's so putting a nuclear ship in Space isn't the treat it was in the 70's, plus we can put more mass into space now, so things like shielding are less costly.